1. Field of the Invention
The present invention relates to an environmental abnormality detection apparatus, in particular an apparatus for detecting a fire, gas leakage, a burglar, and the like.
2. Description of the Prior Art
Various types of environmental abnormality detection devices are known which include detectors specifically designed to monitor the presence of a particular environmental abnormality, such as the presence of a fire, gas, or unauthorized personnel.
A basic arrangement for a widely used fire detector is disclosed in Japanese Patent Publication No. 47-32397. The basic structure is reproduced as FIG. 3 herein. In this fire detector, a closed ion chamber 4 is connected in series with an open ion chamber 8. Smoke cannot enter the chamber 4, which has a pair of electrodes 1 and 2 and a radiation source 3. Smoke can enter the chamber 8, which has a pair of electrodes 5 and 6 and a radiation source 7. The ion chambers 4 and 8 in combination with a field effect transistor 9 form a smoke detector which detects changes in potential at a series connection point between the chambers 4 and 8. When smoke enters the open ion chamber 8, the smoke particles capture ion particles which have been ionized by the radiation source 7 thereby reducing the ion current flowing between the electrodes 5 and 6, and correspondingly increasing the impedance between those electrodes. The transistor 9 detects this increase in impedance as a change in potential. When the potential change thus detected by the transistor 9 exceeds a predetermined potential, set by a Zener diode 10, an SCR 11 is triggered to short-circuit power source/signal lines 12 and 13, thereby informing a receiver (not shown) of the detection of smoke.
As described above, this conventional fire detector uses an SCR or the like at the final stage so that operation is continued after an event such as the presence of smoke disappears. It is sometimes necessary, however, to cancel the short across the power source/signal lines immediately after the event disappears, and to provide a signal or some type of information indicating the disappearance of the event. In this case, the SCR cannot be simply replaced with a switching element such as a transistor which does not have the signal-holding functon of an SCR. This is because the power source/signal lines are shorted during a detection operation as described above. Since during this time power is no longer supplied to the electronic circuit, disappearance of the cause of the alarm signal cannot be monitored.